The Shadow Cipher by Laura Ruby

It was 1798 when the Morningstarr twins arrived in New York with a vision for a magnificent city: towering skyscrapers, dazzling machines, and winding train lines, all running on technology no one had ever seen before.

Fifty-seven years later, the enigmatic architects disappeared, leaving behind for the people of New York the Old York Cipher—a puzzle laid into the shining city they constructed, at the end of which was promised a treasure beyond all imagining. By the present day, however, the puzzle has never been solved, and the greatest mystery of the modern world is little more than a tourist attraction.

Tess and Theo Biedermann and their friend Jaime Cruz live in a Morningstarr apartment—until a real estate developer announces that the city has agreed to sell him the five remaining Morningstarr buildings. Their likely destruction means the end of a dream long held by the people of New York.

And if Tess, Theo, and Jaime want to save their home, they have to prove that the Old York Cipher is real. Which means they have to solve it.